Oscillating hot wire or hot film flow sensor

ABSTRACT

This invention is a flow measurement device that has high spatial (less than 1.0x1.0 mm 2 ) and temporal resolution (greater than 10s to 100s kHz) to measure flow properties in unsteady and direction-reversing conditions. The present invention can have an oscillating substrate, hot wire prongs, a hot wire attached to the hot wire prong, sensor leads from the prongs to a constant temperature anemometry circuit (CTA), means for the oscillating substrate to oscillate the substrate at a frequency greater than a characteristic cycle frequency of the flow to be measured, at a frequency less than a CTA bandwidth frequency, and such that a frequency and amplitude (A w ) of oscillation are sufficiently large to be detected, and means to obtain two measurements during an oscillation cycle when the hot wire is at its maximum oscillation velocity. Alternatively, the prongs can be eliminated and a hot wire or hot film can be directly applied to the oscillating substrate.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to a U.S. provisional patentapplication Ser. No. 60/340,649 titled, “Oscillating Hot-Wire orHot-Film Flow Sensor,” filed 30 Oct. 2001. The entire disclosure of Ser.No. 60/340,649 is incorporated herein by reference in its entirety.

FIELD OF INVENTION

[0002] The present invention relates generally to flow measurementtechniques, and specifically to an oscillating hot wire sensor formeasurement of unsteady, direction-reversing flow velocity andwall-sheer stress in aerodynamic and hydrodynamic applications.

BACKGROUND OF INVENTION

[0003] A common method to indirectly measure unsteady surface shear usesa “hot” wire or “hot” film on the surface and is well known in the art.Surface shear is a tangential force exerted on a surface (wall) causedby flow moving over the surface. This method determines surface shear bymeasuring cooling of the wire or film at the surface and comparing it toknown cooling velocity values. This technique is the only known methodto measure high-frequency surface shear fluctuations. In addition toonly indirectly measuring surface shear, the “hot” wire or “hot” filmmethod does not measure directional changes of the flow. To remedy theindirect nature and directional ambiguity of the prior art,surface-shear measurements via shear balances (a shear sensor thatincludes a strain gauge or other methods known in the art) mounted flushwith the surface were developed. These devices would directionallydeflect under the action of shear. Although capable of detectingdirection and amount of surface shear, the bulkiness of their floatingelements made them only useful for time-averaged measurements over alarge area, rather than at a single point (never less than 1.0×1.0 mm²).

[0004] The emergence of micro electro mechanical systems (MEMS)technology generated considerable hope in constructing micron-sizedfloating elements, or shuttles, with a sensing area less than 0.5×0.5mm² and a bandwidth of tens and even hundreds of kHz (i.e., much greatertemporal resolution).

[0005] The excitement over MEMS floating elements resulted in a numberof attempts to construct high precision directional shear flow sensors.Although some success was achieved in constructing and testing the MEMSfloating elements, one problem was soon realized. The extremely smallarea on which the surface shear acts can only produce Angstrom-sizedeflections. Thus, in order to maximize the deflection, the shuttlesupport could only be a few microns wide. This rendered these sensorsfragile and, for all practical purposes, only useable by their makersunder highly controlled conditions. Additionally, the minute deflectionsof the floating element within the shuttle did not seem to producesufficient signal-to-noise ratio, particularly when using capacitivepickups known in the art for detecting the deflection.

[0006] Other attempts to remove directional ambiguity from hot wiremeasurements included the use of “pulsed-wire” anemometers previouslyused for velocity measurements in separated flows. This technologyapplied to measuring surface shear stress was later developed using asensor having a central heating wire surrounded by upstream anddownstream cold wires. A central wire, typically oriented at 90 degreeswith respect to the sensor wires, is heated periodically. Fluid velocityis measured from the time of heating the central wire until a change intemperature is detected by one of the cold wires (time of flight). Flowdirection (forward or reverse) is determined by which cold wire changestemperature.

[0007] Unfortunately, there are several difficulties and limitationsusing this pulsed anemometry technology. First, to avoid thermaldiffusion effects, the sensing volume size is typically no less than oneto two millimeters. This limits sensor spatial resolution. This sensorseparation limits the frequency response to tens, or a few hundred Hz atbest (i.e., low temporal resolution). Second, in flows with largevelocity gradients, such as near surfaces (walls), the measurements mustbe corrected using constants. Finally, pulsed hot wires requireelaborate and careful calibration. Again, this limits their applicationsince they are impractical for applications involving arraymeasurements.

[0008] A different variation on pulsed anemometry also known in the artuses three parallel wires to measure the fluid velocity in aone-dimensional pulsating flow such as in a pipe. In this approach, acentral wire is operated as a conventional constant-temperature sensorand used to measure the magnitude of the velocity. Flow direction isindirectly determined by incorporating the two outside wires in oppositelegs of a Wheatstone bridge to form a thermal tuft, known in the art, onthe wall under a re-attachment zone of a backward facing step. Althoughthis method overcomes some of the disadvantages of the time of flighttechnique, the frequency bandwidth remains limited to tens or a fewhundred Hz due to separation of the thermal tuft sensors and theirthermal inertia.

[0009] Thus, there remains a need to develop a flow measurement devicethat has high spatial (less than 1.0×1.0 mm²) and temporal resolution(greater than 10s to 100s kHz) to measure fluid flow properties inunsteady and direction-reversing fluid flows.

SUMMARY OF THE INVENTION

[0010] Accordingly, a feature of the present invention is to provide ahigh spatial and temporal resolution sensor for measurements of unsteadydirection-reversing surface shear stress produced by a fluid flow inaerodynamic and hydrodynamic applications. Another feature of thepresent invention is to provide a sensor that can be used for flowvelocity measurements in direction reversing flows.

[0011] Specifically, the present invention is a flow measurement devicethat has high spatial (less than 1.0×1.0 mm²) and temporal resolution(greater than 10s to 100s kHz) to measure fluid flow properties inunsteady and direction-reversing fluid flows.

[0012] The present invention can have, in a preferred embodiment, anoscillating hot wire sensor to measure flow, having an oscillatingsubstrate, at least two conductive hot wire prongs having a first endattached to the oscillating substrate and a second end extending abovethe oscillating substrate, a hot wire attached and stretched across thehot wire prong second ends; sensor leads comprising first sensor leadends and second sensor lead ends, the first sensor lead ends attached tothe first hot wire ends; constant temperature anemometry (CTA) circuitryconnected to the second sensor lead ends; means for the oscillatingsubstrate to oscillate the substrate at a frequency greater than acharacteristic cycle frequency of the flow to be measured, at afrequency less than a CTA bandwidth frequency, and such that a frequencyand amplitude (A_(w)) of oscillation are sufficiently large to bedetected; and means to obtain two measurements during an oscillationcycle when the hot wire is at its maximum oscillation velocity.Alternatively, the prongs can be eliminated and a hot wire or hot filmcan be directly applied to the oscillating substrate.

[0013] The oscillating hot wire prong can extend in the range of 5 to 10microns above the substrate or protective cover (if used), the CTAbandwidth is in the range of 20-40 kHz, the frequency greater than acharacteristic cycle frequency of the flow to be measured is greaterthan 5 kHz.

[0014] The means for the oscillating substrate can use piezoelectric ormicro electro mechanical systems (MEMS) technology.

[0015] A protective cover with openings that covers the oscillatingsubstrate, the openings allowing hot wire prong second ends to extendthrough the protective cover can be added.

[0016] In an alternate embodiment, a first pressure sensor is addedupstream of the flow and a second pressure sensor is added downstream ofthe flow, whereby a pressure gradient can be determined.

[0017] Briefly summarized, the invention provides an oscillating hotsensor for measuring fluid flow which includes a substrate adapted to bemounted for oscillation in a fluid flow passage having an axis forreversible fluid flow, the axis extending between a first flow directionand a second, opposite flow direction. A hot sensor element such as ahot wire or hot film is carried by the substrate and is connected byleads to constant temperature anemometry circuitry. Means such as apiezoelectric or MEMS device is provided for oscillating the substratebetween the first and second fluid flow directions. Means are embodiedin the circuitry for obtaining a first velocity measurement in anoscillation cycle during flow in said first flow direction and a secondvelocity measurement during flow in said second flow direction and,means are also embodied in the circuitry for comparing signalsrepresentative of the first and second velocity measurements.

[0018] Other features of the present invention will become more apparentto persons having ordinary skill in the art to which the presentinvention pertains from the following description taken in conjunctionwith the accompanying figures.

BRIEF SUMMARY OF THE FIGURES

[0019]FIG. 1 illustrates a typical hot wire response curve.

[0020]FIG. 2 illustrates a side view of the oscillating hot wire sensorof the present invention.

[0021]FIG. 3 illustrates an end view of the oscillating hot wire sensorof the present invention.

[0022]FIG. 4 illustrates a response curve of the oscillating hot wiresensor of the present invention.

[0023]FIG. 5 is a diagram representing the operating circuit used in theapparatus of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention is a high spatial and temporal resolutionsensor for measurements of unsteady direction-reversing surface (wall)shear stress produced by a fluid flow in aerodynamic and hydrodynamicapplications. The flow measurement device of the present invention hashigh spatial (less than 1.0×1.0 mm²) and temporal resolution (greaterthan 500 Hz up to 100s kHz) to measure fluid flow properties in unsteadyand direction-reversing fluid flows.

[0025] The surface shear sensor of the present invention measures fluidflow velocity (V) with a hot wire very close to the surface. If the wireis located within the region where the velocity varies linearly withdistance from the wall, the surface shear stress (τ_(w)) can beestimated from the velocity measurement as:

τ_(w) =μdV/dy(y=0)≈μV/δy

[0026] where μ is the absolute viscosity, y is the direction normal tothe wall, and δy is the distance from the wall to the hot wire. Thistechnique, known in the art, measures shear stress in flows where alocal velocity vector does not reverse direction.

[0027]FIG. 1 illustrates a typical hot wire response curve. Theinability of the approach to measure the flow velocity (shear stress)and direction can be seen from a typical hot wire response curve 54.FIG. 1 has as its y-axis hot wire output voltage E 50 and flow velocityV 52 as its x-axis. At velocity −V 56 there is a corresponding output ofE_(−V) 60. Similarly at velocity V 58 there is a corresponding output ofE_(V) 62. As illustrated in FIG. 1, the hot wire output voltage (E) 50remains the same if the velocity magnitude is unchanged but itsdirection is reversed, (i.e., E_(−V) 60 equals E_(V) 62). This givesrise to the classical directional ambiguity of hot wires.

[0028] The present invention can overcome this directionalinsensitivity, by mounting a near-surface sensor 75 on an oscillatingsubstrate 90, as shown in FIGS. 2 and 3. FIG. 2 shows a side view of thepresent invention, used in measurement of a direction-reversing fluidflow 80, while FIG. 3 shows an end view of the present invention. In theillustrated embodiment, an oscillating substrate 90, attached throughvarious means known in the art to a surface 48, is covered by anoptional protective cover 86. At least two conductive hot wire prongs 84having a first end 44 and second end 42 are fixedly attached to theoscillating substrate 90, and extend through openings 46 in theprotective cover 86 in the range of 5 to 10 microns above the protectivecover 86. A hot wire 82 is attached and stretched across the hot wireprongs 84 second ends 42 Alternatively, one skilled in the art couldsubstitute the hot wire 82 and the hot wire prongs 84 with a hot wire orhot film mounted directly on the oscillating substrate 90. The hot wireprongs 84 extend a distance δy 88 above the surface of the optionalprotective cover 86. Sensor leads 92 having first sensor lead ends 38and second sensor leads 40, the first sensor lead ends 38 attach to thefirst hot wire prong end 44; and a constant temperature anemometrycircuitry 94, known in the art attach to the second sensor lead ends 40.

[0029] The oscillation frequency of the new sensor, to be referred to asOHW (or oscillating hot wire), should be higher than any characteristicfrequency in the flow. In this manner, two measurements of the samevelocity can be made with the hot wire 82 moving with and against theflow 80 during the oscillation cycle. Each of the two measurements ispreferably acquired while the hot wire 82 is at its maximum oscillationvelocity (i.e., at the mid-stroke of oscillation).

[0030] To understand how the two measurements per oscillation cycle canbe used to yield the local velocity magnitude and direction, considerthe following analysis, illustrated in FIG. 4. FIG. 4 shows an y-axishot wire output voltage E 100 and an x-axis flow velocity 102. A hotwire response curve is shown at 104. Let “V” be the magnitude of thelocal velocity and “v” be the maximum oscillation velocity of the wire.Also, assume the forward motion of the hot wire 82 oscillation is in thepositive flow-velocity direction (first flow direction). When the localvelocity vector is positive, the measurement with the hot wire 82 movingin the forward (with the flow) direction yields a voltage E_(for-POS)118 in response to velocity V−v 110 (since the flow velocity relative tothe hot wire 82 is reduced by an amount equal to the wire velocity).Similarly, the hot wire 82 output voltage for the measurement in thebackward direction (E_(back-POS)) 120 would result from a flow velocityof V+v 112 (because the hot wire 82 is now moving opposite to the flowdirection). Since E_(back-POS) 120 results from a higher flow velocity,it will be larger than E_(for-POS) 118. If, on the other hand, the flowvelocity is in the negative direction, E_(back-NEG) 116 will be lessthan E_(for-NEG) 114. Thus, the direction of the local velocity vector(whether it is positive or negative) can be determined from the sign ofthe difference: E_(back)−E_(for).

[0031] When selecting the hot wire 82 oscillation frequency (f_(w)),there are three major constraints that must be considered.

[0032] First, the oscillation frequency must be higher than anycharacteristic frequency in the flow, such that the flow would be“frozen” during the time between the two successive measurements. Afrequency of 5 kHz or higher should be sufficient for most laboratoryexperiments and a large number of applications.

[0033] Second, the measured oscillation frequency bandwidth must be lessthan the bandwidth of the hot wire 82 in order for the hot wire 82 tokeep pace with the velocity oscillation. This hot wire 82 band width isdetermined by the CTA circuitry 94 used to operate the sensor. A typicalCTA 94 bandwidth is in the range of 20-40 kHz.

[0034] Third, the frequency and amplitude (A_(w)) of oscillation shouldbe such that the maximum wire velocity (v=2πA_(w)f_(w)) is sufficientlylarge to be detected. For example, if A_(w)=1 μm and f_(w)=5000 Hz, thenv=0.0314 m/s. Such a velocity disturbance should be easily detected ifthe local velocity is 1 m/s or smaller. If one assumes the hot wire 82location to be a few microns (preferably between 5 to 10 microns, butfor this example, 5 microns) above the surface 48, then the maximummeasurable shear stress in airflow (corresponding to V=1 m/s andμ_(air)=1.8×10⁻⁵ N Sec/m²) is 3.6 Pa. This value can be increased byincreasing the oscillation amplitude and/or frequency (while satisfyingthe second constraint).

[0035] Although the required frequency of oscillation (a few kHz) is toohigh to achieve with a conventional mechanical element, the desiredfrequency (and amplitude) values are well within the range attainable byresonant structures fabricated using micro electro mechanical systems(MEMS) technology or piezoelectric technology. Furthermore, MEMStechnology has already been used to fabricate resonant structures aswell as conventional hot wire sensors.

[0036] Therefore, the proposed sensor can be realized using MEMS orpiezoelectric technology to fabricate an integratedoscillating-substrate/hot wire system that is capable of oscillating atfrequencies up to tens of kHz (and more if CTA technology is improved toaccommodate wider bandwidths) and amplitudes up to tens of microns. Atsuch high frequency of oscillation the new sensor of the presentinvention will be capable of measuring the unsteady direction-reversingshear stress with a bandwidth of a few to tens of kHz. This is atremendous improvement in temporal resolution as compared to pulsed-wireanemometry. Furthermore, the spatial resolution of the hot wire 82 isthat of a conventional hot wire which is typically in the range of 100to 500 μm (compared to the few millimeter sensor size in pulsed-wireanemometry). Finally, the inherent ability of MEMS and piezoelectrictechnology to fabricate sensor arrays will be extremely useful inextending the use of the new sensor to measurement of the surface shearstress distribution over large surface areas.

[0037] To more completely understand the disclosed invention, a briefdiscussion of the linearity of the mean velocity profile in themeasurement zone very close to the wall is necessary. The assumedlinearity is a consequence of conducting measurements at very small yvalues such that a Taylor series expansion of the flow velocity, knownin the art, can be truncated after the first order (linear) term.However, the second-order (y²) term in the expansion may becomesignificant for flows where a strong pressure gradient is present. Insuch a case, the sensor measurements must be corrected for the pressuregradient (or quadratic) effects. The present invention accomplishes thisby integrating the proposed sensor with two pressure sensors 96 just upand downstream of the OHW to measure the pressure gradient. That is, inits more versatile form, the new sensor includes both the oscillatingwire and two pressure sensors 96. Fortunately, MEMS and piezoelectrictechnology have also been used to fabricate pressure sensorssuccessfully. Thus, the full OHW system can be fabricated by integratingall components in the micro fabrication process.

[0038] A preferred electrical circuit used to operate the oscillatinghot wire sensor 75 of this invention is illustrated diagrammatically inFIG. 5. The circuit includes the following components:

[0039] 1. A CTA 94, which is the same unit as that used to operateconventional hot wires. The CTA 94 outputs a voltage E 100 that is afunction of the magnitude of the measured velocity 102.

[0040] 2. An oscillator driver 130, which outputs a sinusoidal(oscillation driving) signal 132 with the desired oscillation frequency,amplitude and power to cause the oscillating hot wire of sensor 75 tovibrate. The signal is also simultaneously fed to a synchronizationcircuit 134.

[0041] 3. Synchronization circuit 134 accepts the hot wire output signal(voltage E) 100 and the oscillation driving signal 132. Thesynchronization circuit 134 then outputs the hot wire output voltage(E_(for-POS)) 118 and 120 (E_(back-POS)) at the two phases of theoscillation driving signal 132 corresponding to the forward and backwardpositions of the sensor 75. That is, for each cycle of oscillation ofthe wire of sensor 75, the synchronization circuit 134 outputsE_(back-POS) 120 and E_(for-POS) 118, only, and the case of flow in thenegative direction output E_(for-NEG) 114 and E_(back-NEG) 116. In FIG.5 readings are shown in the positive flow direction for purposes ofillustration

[0042] 4. A comparator 136, which subtracts E_(for-POS) 118 fromE_(back-POS) 120 and outputs either plus or minus volts for positive andnegative difference, respectively.

[0043] 5. A peak detector 138, determines the larger of E_(for-POS) 118and E_(back-POS) 120 for every oscillation cycle, which is equal toE_(peak) 122.

[0044] 6. A multiplier 140, which multiplies the output of thecomparator 136 (representing either a positive or negative direction ofthe flow) by the larger of E_(for-POS) 118 and E_(back-POS) 120. In thismanner, the output voltage polarity provides the direction of flow 80and the output signal magnitude is a function of the flow velocity.

[0045] Various alterations and changes can be made to the illustratedembodiment of the present invention without departing from the spiritand broader aspects of the invention as set forth in the appendedclaims, which are to be interpreted in accordance with the principles ofpatent law, including the doctrine of equivalence. The embodiment of theinvention in which exclusive property or privileges claimed is definedas follows.

We claim:
 1. An oscillating hot wire sensor to measure flow, comprising:an oscillating substrate; at least two conductive hot wire prongs havinga first end attached to the oscillating substrate and a second endextending above the oscillating substrate; a hot wire attached andstretched across the hot wire prong second ends; sensor leads comprisingfirst sensor lead ends and second sensor lead ends, the first sensorlead ends attached to the first hot wire prong ends; constanttemperature anemometry (CTA) circuitry connected to the second sensorlead ends; means for the oscillating substrate to oscillate thesubstrate at a frequency greater than a characteristic cycle frequencyof the flow to be measured, at a frequency less than a CTA bandwidthfrequency, and such that a frequency and amplitude (A_(w)) ofoscillation are sufficiently large to be detected; and means to obtaintwo measurements during an oscillation cycle when the hot wire is at itsmaximum oscillation velocity.
 2. The oscillating hot wire sensor ofclaim 1 wherein the hot wire prong second end extends in the range of 5to 10 microns above the protective cover.
 3. The oscillating hot wiresensor of claim 1, wherein the CTA bandwidth is in the range of 20-40kHz.
 4. The oscillating hot wire sensor of claim 1, wherein thefrequency greater than a characteristic cycle frequency of the flow tobe measured is greater than 5 kHz.
 5. The oscillating hot wire sensor ofclaim 1, wherein the means for the oscillating substrate usespiezoelectric technology.
 6. The oscillating hot wire sensor of claim 1,wherein the means for the oscillating substrate uses micro electromechanical systems (MEMS) technology.
 7. The oscillating hot wire sensorof claim 1, further comprising a protective cover with openings thatcovers the oscillating substrate, the openings allowing hot wire prongsecond ends to extend through the protective cover.
 8. The oscillatinghot wire sensor of claim 1, further comprising a first pressure sensoris added upstream of the flow and a second pressure sensor is addeddownstream of the flow, whereby a pressure gradient can be determined.9. An oscillating hot wire sensor to measure flow, comprising: anoscillating substrate; a hot wire attached and stretched across theoscillating substrate; sensor leads comprising first sensor lead endsand second sensor lead ends, the first sensor lead ends attached to thehot wire ends; constant temperature anemometry circuitry connected tothe second sensor lead ends; means for the oscillating substrate tooscillate the substrate at a frequency greater than a characteristiccycle frequency of the flow to be measured, at a frequency less than aCTA bandwidth frequency, and such that a frequency and amplitude (A_(w))of oscillation are sufficiently large to be detected; and means toobtain two measurements during an oscillation cycle when the hot wire isat its maximum oscillation velocity.
 10. An oscillating hot film sensorto measure flow, comprising: an oscillating substrate; a hot filmattached and stretched across the oscillating substrate; sensor leadscomprising first sensor lead ends and second sensor lead ends, the firstsensor lead ends attached to the hot film ends; constant temperatureanemometry (CTA) circuitry connected to the second sensor lead ends;means for the oscillating substrate to oscillate the substrate at afrequency greater than a characteristic cycle frequency of the flow tobe measured, at a frequency less than a CTA bandwidth frequency, andsuch that a frequency and amplitude (A_(w)) of oscillation aresufficiently large to be detected; and means to obtain two measurementsduring an oscillation cycle when the hot film is at its maximumoscillation velocity.
 11. An oscillating hot sensor for measuring fluidflow comprising: a substrate adapted to be mounted for oscillation in afluid flow passage having an axis for reversible fluid flow, said axisextending between a first flow direction and a second, opposite flowdirection; a hot sensor element carried by said substrate; constanttemperature anemometry circuitry; leads connecting said hot sensorelement to said circuitry; means for oscillating said substrate betweensaid first and second fluid flow directions along said axis; meansembodied in said circuitry for obtaining a first velocity measurement inan oscillation cycle during flow in said first flow direction and asecond velocity measurement in said cycle during flow in said secondflow direction; and a comparator operatively connected to said circuitryfor comparing said first and second velocity measurements.
 12. A sensoraccording to claim 11 wherein said hot sensor element comprises a hotwire supported on a pair of prongs carried by said substrate.
 13. Asensor according to claim 11 further comprising a multiplier configuredto multiply the output of said comparator.
 14. A sensor according toclaim 11 wherein an oscillator driver outputs a sinusoidal signal tosaid sensor and to a synchronization circuit which also receives theoutput of said anemometry circuitry.
 15. A sensor according to claim 14wherein said synchronization outputs voltage signals representingvoltages at said first and second flow directions to a comparator whichsubstracts said voltages whereby flow direction and flow velocity can bedetermined.